This invention relates to an improved exhaust sampler and control means, and more particularly to an apparatus for measuring the gaseous constituents of exhaust utilizing means for measuring the exhaust or exhaust/air mixture flow including a calibrated subsonic venturi, and means for controlling the flow rate of dilution air or exhaust/air mixture.
Under present day federal regulations, the exhaust emissions from motor vehicles must not exceed specified values of certain constituent contaminants, as set forth in the Code of Federal Regulations. See Title 40 of the Code of Federal Regulations, Parts 81-99, Subparts A, B, D, E, F, G, K and N. See also Kaufman U.S. Pat. No. 3,699,814. The presence of such standards has made it imperative that the exhaust emissions from vehicle engines be tested and analyzed to determine the relative amount of certain constituents therein. Much effort has gone into the development of equipment for use in this field of exhaust gas sampling, and it is now known to deliver exhaust gases from an internal combustion engine at an accurately controlled flow rate through a test apparatus for purposes of determining and analyzing the relative amounts of constituents therein. The general scheme of such testing is to add dilution air to the exhaust gases. The total volume of the mixture of exhaust and dilution air must be measured. A continuously proportional sample of volume must be collected and is stored for subsequent analysis of constituents such as hydrocarbons, carbon monoxide, and NO.sub.x. Mass emissions are determined from the sample concentrations and total flow over the test period.
One such system for analyzing samples from exhaust gases is set forth in U.S. Pat. No. 3,699,814 to Kaufman entitled, "Gas Sampler," issued Oct. 24, 1972. The Kaufman patent, the disclosure of which is incorporated herein by reference, taught a much improved gaseous exhaust emission sampler which replaced the constant displacement pump of prior systems with a critical flow venturi and centrifugal blower for metering the diluted exhaust emissions at a constant volume flow.
Another system utilizes a pair of critical flow venturis for proportional sampling. An example of such a system is set forth in U.S. Pat. No. 3,817,100. In another such system, a downstream pump produces a sufficient vacuum on the bulkstream critical flow venturi exit so that the bulkstream mixture is flowing at sonic velocity, a condition which limits the bulkstream mixture to a constant mass flow rate at a given set of upstream temperature and pressure conditions measured at the bulkstream critical flow venturi inlet. A sample is extracted from the dilute bulkstream flow through another critical flow venturi in close proximity to the bulkstream critical flow venturi so that the venturis are operating under the same inlet pressure and temperature conditions. This sample critical flow venturi operates in connection with a downstream pump in the sampling line to create sonic flow, and thereby a constant mass flow rate at the measured upstream temperature and pressure conditions. Thus, the sample critical flow venturi extracts a sample for analysis at a flow rate proportional to the bulkstream flow rate.
Although the flow rate controlled by a critical flow venturi will vary a small amount due to changes in the venturi inlet temperature and pressure, if the sample venturi and bulkstream venturi are operating at critical flow rate conditions at identical inlet pressure and temperature conditions, the sample flow rate is extracted in proportion to the bulkstream flow rate.
Although this proportional sampling system using two critical flow venturis has certain advantages, it has a disadvantage in that it does not provide for active control of the bulkstream or sample flow rates to allow static and dynamic sampling of the bulkstream mixture. Consequently, such prior art systems cannot readily accommodate the testing of different sized internal combustion engines which generate substantially different exhaust flow volumes without using different bulkstream critical flow venturis.
It is, therefore, a principal object of this invention to provide an apparatus for sampling the emission content of exhaust from an exhaust source wherein a wide variety of flow rates may be selected and controlled between or during the test phases.
It is another object of this invention to provide an apparatus for sampling the emission content of exhaust from an exhaust source wherein the apparatus has a constant flow rate operating mode and a variable flow rate operating mode that provides a means for maintaining a constant dilution ratio with changing bulkstream or exhaust flow rate.